Colliding artillery shells

An artillery crew demonstrates its skill by firing a shell at an angle of70.0deg and then lowering the gun barrel and firing a second shell at a smaller angle of15.0deg in such a way that the two shells collide in midair. The initial velocity of the shells is320.m/s.

(Sometimes, the shells actually collide below the level of the gun barrel: that is OK, because the gun is at the edge of a cliff.

Find the time elapsed between the two firings, in seconds. Neglect air resistance.

Hint: A collision means that both horizontal and vertical displacements are equal.